Recently, burners have been developed that use oxygen or oxygen-enriched air to support combustion of a fuel in a burner known as an oxy-burner. Oxy-burners are compact and produce typically small flames with a high power output. In conventional heating and melting operations, several different types of fuels, such as natural gas, propane, coal gas, oil, and the like, can be used to obtain the high temperatures necessary to change the furnace charge from a solid to a pre-heated or molten state. In an oxy-burner, substantially pure oxygen, generally 80% oxygen or higher, is mixed with the fuel gas to produce extremely high flame temperatures. The high flame temperatures can rapidly heat or melt the furnace charge. Rapid melting is particularly beneficial in the manufacture of iron and steel. Additionally, oxy-burners are widely used in various metallurgical plants to reduce melting time and the total energy necessary to bring the metallurgical charge to a molten state.
Operation of an oxy-burner necessarily requires that a supply of oxygen is readily available to operate the burner. Typically, an on-site oxy generation plant, such as vacuum or pressure swing absorption units, or cryogenic air separation units are maintained in proximity to the oxy-burner. During burner operation, a continuous, uninterrupted supply of oxygen is necessary to avoid production losses and potential damage to the burner system if the supply of oxygen is interrupted. In certain non-water cooled oxy-burners, metallic parts can be damaged by furnace radiation unless the burner is pulled out of service, or cooled with auxiliary cooling air or water that is circulated to the burner nozzles.
To limit the possibility of production losses and burner damage, metallurgical plant operations typically provide a liquid oxygen supply tank to serve as a back-up oxygen supply. The liquid oxygen supply requires continuous replenishing to compensate for evaporation losses. Because of the relatively high cost of maintaining a liquid oxygen back-up supply, many metallurgical operations fail to store sufficient back-up oxygen to meet their entire needs during a disruption in the primary oxygen supply. Additionally, because of space limitations, back-up oxygen supply tanks may not hold enough oxygen to operate the burner for the required operation.
An alternative to on-site oxygen storage is to provide a back-up air supply system. In the event of a disruption in the oxygen supply, the oxy-burner can be operated as an air-fuel burner. Although the operation of an oxy-burner with a back-up air supply system maintains burner operation, the air must be free of lubricating grease, oils, and other contamination to avoid damaging the oxy-burner. The requirement for an extremely clean back-up air supply limits the back-up air supply system to the use of dedicated air lines and delivery equipment. The need to use dedicated equipment, such as compressors, blowers, piping performance, flow controls, and the like increases the overall capital cost of the furnace combustion system. Further, the dedicated air supply equipment requires that a relatively large amount of space be available for the installation of equipment that is used only intermittently. Moreover, after operating an oxy-burner from a back-up air system, the oxy-burner must be removed from the furnace and thoroughly cleaned to ensure that the burner has not been contaminated by air operation.
Although oxy-burners offer a convenient means of obtaining high flame temperatures for operation of metallurgical furnaces, economic operation of the furnace requires a reliable and economic method of operation in the event of a loss in the primary oxygen supply. The economic and safety considerations in the operation of a metallurgical furnace require that a back-up firing system be safe, fast, functional and cost effective. Accordingly, a need exists for an improved back-up oxy-burner firing system and method of operation.